1. Field of the Invention
This invention relates to a display monitor which receives commands from an external device such as a computer and controls a display position and a display size of the display monitor.
2. Description of the Prior Art
FIG. 8 is a block diagram showing a configuration of a conventional display monitor which has a communication function between a computer and the display monitor. In this figure, numeral 1 denotes a computer, numeral 2 denotes a display monitor which can control a display position and a display size of the display monitor based on a command received from the computer 1, numeral 3 denotes a Central Processing Unit (CPU) provided in the computer 1, numeral 4 denotes a video interface which receives data from the CPU 3 and sends image signals to the display monitor 2, numeral 5 denotes a video cable for providing the image signals from the computer 1 to the display monitor 2, numeral 6 denotes a serial interface provided in the computer 1, which performs a serial data communication between the display monitor 2 and the computer 1, numeral 7 denotes an interface cable for sending commands from the serial interface 6 to the display monitor 2. Next, the elements in the display monitor 2 are explained. Numeral 8 denotes a serial interface which performs a serial communication between the computer 1 and the display monitor 2 via the interface cable 7, numeral 9 denotes a monitor such as a Cathode Ray Tube (CRT), numeral 10 denotes a monitor circuit which drives the monitor 9 according to the video signals supplied via the video cable 5, numeral 11 denotes an Electrically Erasable and Programmable Read Only Memory (EEPROM) which stores data necessary for controlling the monitor circuit 10, numeral 12 denotes a Random Access Memory (RAM) for storing adjustment data of the monitor circuit 10 for currently displayed image, numeral 13 denotes a monitor control CPU which outputs adjustment data stored in the RAM 12 to the monitor circuit 10 via a Digital to Analog (D/A) converter which is not shown in the figure. The monitor control CPU 13 controls the monitor circuit 10 according to the commands received by the serial interface 8.
The display monitor 2 receives commands of the CPU 3 by performing a communication between the serial interfaces 6 and 8, and the commands are sent to the monitor control CPU 13. The monitor control CPU 13 controls the monitor circuit 10 according to the commands so as to enlarge the screen size of the monitor 9 or to change the screen position of the monitor 9.
FIG. 9 is a sequence diagram of data communication between the computer 1 and the display monitor 2 in the case where the screen size is enlarged, for example. As shown in this figure, the CPU 3 of the computer 1 sends a command to the display monitor 2, which indicates that the screen size is to be enlarged. In this case, the serial interface 8 sends an acknowledge signal back to the computer 1. The monitor circuit 10 increases the voltage applied to a deflection circuit by a predetermined amount in order to enlarge the screen size of the monitor 9 under control of the monitor CPU 13. In this prior art, the CPU 3 sends such a command several times to obtain a desired screen size.
Adjustment data for parameters of the monitor circuit 10 corresponding to the currently displayed image are stored in the RAM 12. Monitor control CPU 13 sets the adjustment data in RAM 12 corresponding to the frequencies of vertical and horizontal synchronous signals of the supplied video signals for example. Further, the CPU 13 transfers data in the EEPROM 11 to the RAM 12. Moreover, the adjustment data are renewed when adjustment of the screen is performed. Data are stored in the EEPROM 11 corresponding to parameters such as the frequencies of synchronous signals when the display monitor is produced at a factory.
The above explained conventional monitor display has the following drawbacks. Firstly, when enlarging the screen size of the monitor 9, it is necessary to supply a plurality of commands to obtain a desired screen size. Secondly, it is impossible to know the parameter values of the display monitor 2 from the outside of the display monitor 2 and it is also impossible to confirm whether the parameters of the display monitor 2 are properly set based on a received command or not. Thirdly, data values stored in the EEPROM 11 cannot be known and cannot be changed at all once the display monitor is completed.